1. Field of the Invention
This invention relates to pressure-sensitive record materials and particularly to solvents for colorless dyes used in preparing such materials.
2. Description of the Prior Art
Conventional pressure-sensitive record materials comprise in combination marking fluid or dye and a solid coreactant which are deposited in substantially continuous coatings upon a dye carrying sheet and a dye receiving sheet, and which are separated by a physical barrier which is eliminated by the application of pressure. One such record material comprises a first sheet material containing a substantially continuous covering of pressure rupturable capsules containing as a marking fluid a solution of a chromogenic material in a solvent, and a second sheet material containing in apposition to the capsules on the first sheet a continuous coating of a solid acidic sensitizing material which is reactive with the marking liquid to form a colored reaction product. The capsules containing the marking fluid are sufficient in number and volume to yield a continuous image pattern when the marking fluid is expelled from the capsules by pressure applied in an image pattern.
The marking fluid contained in the capsules of the first sheet can be any of a variety of liquid compositions provided they produce a colored mark when allowed to contact the solid coreactant. Generally desirable properties of the marking fluid are that it be easily encapsulated by conventional techniques, that it have good shelf life in the encapsulated form, and that it be stable a moderately elevated temperatures. It is also important that the mark produced as the result of the reaction between the marking fluid and the solid coreactant develop rapidly, be fade resistant and be resistant to bleeding or feathering as a result of capillary action or other surface phenomena.
The marking fluid is preferably a solution of a solvent and a colorless or substantially colorless chromogenic material which develops color upon contact and reaction with the solid coreactant or sensitizing material. Such marking fluids have the advantage of not discoloring hands, clothing, or other surfaces if accidentally ruptured thereon.
Solid coreactants or sensitizing materials for such marking fluids include finely divided acidic compounds which are also colorless or nearly colorless in their natural form. Commonly used materials include organic polymers and inorganic clays which are applied to the paper surface in any suitable paper coating binder material such as starch, casein, polymer, or latex.
The solvent functions to provide a carrier for the chromogen and a medium for the reaction between the chromogen and the acidic sensitizing material. As a general practice, the chromogen is dissolved in the solvent to form a solution which may be encapsulated and applied as a coating to one surface of the record paper. The solvent must be capable of holding the chromogen in solution within the capsule, of carrying the marking liquid to the sensitized paper when the capsule is ruptures, and of promoting or at least not inhibiting color development with the solid coreactant. In addition, since inadvertent rupture of the capsule is possible by careless handling, the solvent must be an innocuous material which is not injurious to skin, clothing or environment.
The solvent is an important factor in determining the performance of the record transfer material in terms of stability of the paper to heat and storage time, rate of color development, extent of color development, and durability of image. The prior art, however, has paid little attention to the subject of solvents and has concentrated instead on improvements in chromogens and sensitizing materials utilizing solvents taken from a limited number of compounds such as petroleum oils and distillates, toluene, perchloroethylene, xylene, chlorinated paraffins, dioctyl phthalate and methyl salicylate. Chlorinated diphenyls, alkylated diphenyls, and hydrogenated terphenyls for example have been suggested in U.S. Pat. No. 2,646,367 as a substitute for petroleum oil in a system wherein the petroleum oil functioned as an inert carrier for a colorless dye. Although some of these past applications have given fair results, only the polychlorinated biphenyls have proved sufficiently effective in providing dark and permanent colors to be of commercial value as a dye solvent, and the full potential of the dye solvent as a positive contributor to the performance of the record transfer material has not heretofore been realized. Certain of the polychlorinated biphenyls have now been determined to be biologically stable and potentially harmful to the environment, and thus undesirable for continued use in this application.
It is accordingly an object of the present invention to provide solvents and combinations of solvents which are superior to those presently used in record transfer materials and which make a positive contribution to the performance of the paper. Another object of this invention is to provide a dye solvent which is free of polychlorinated biphenyls. Further objects of the invention will be apparent from the ensuing description and examples.